The present invention relates to imagers used to detect light or ionizing radiation. In particular, the present invention relates to imagers with imaging arrays designed to reduce image-degrading current.
Radiation imagers comprised of semiconductor diode arrays are commonly used to detect ionizing radiation. The semiconductor diode arrays can be arranged with scintillators to form detector pixels and are used for X-ray Computed Tomography (CT).
Leakage currents in imagers (e.g., X-ray imagers) can have a significant impact on noise and the stability of the dark offset. Leakage current is a source of additional noise and generates offsets which disadvantageously drift with temperature. The dark offset is the signal produced by the imaging array without any radiation. A value for the dark offset is stored and subtracted from the radiation signal data. Changes of the dark offset in time, due to, for example, ambient temperature changes or temperature changes in the imager during operation, are undesirable.
Although leakage currents from defects in the components of such imagers have dramatically decreased in recent years due to improved fabrication processes, leakage currents still result from at least four sources. The first source of leakage current results from cutting semiconductor chips with the imaging array thereon from semiconductor wafers. The cut edge is typically not well passivated, and hence tends to be a significant source of leakage current. The second source of leakage current occurs where adjacent pixel diodes of the array are maintained at very small bias voltages (i.e., less than 50 mV) and at slightly different reverse bias voltages, resulting in a bipolar current between adjacent light-sensitive imaging devices. This bipolar current is reduced to zero for larger reverse diode bias (i. e., much larger than 50 mV). The use of large ( greater than  greater than 50 mV) reverse bias has two advantages in imaging applications. First, as described above, it can reduce one component of undesirable leakage current, which is a source of offset and offset thermal drift in imaging applications. Second, with reverse bias on the diode, and one terminal of the diode an open circuit, signal charge will produce a reduction of diode voltage. This charge can then be readout by connecting the diode to an amplifier using a switch device. This permits analog multiplexed readout of charge on multiple diodes in the array using a single amplifier. Such larger reverse diode bias, however, increases the other sources of leakage current. Hence many detector arrays are operated at near zero bias voltage In the present invention the diode bias is large ( greater than  greater than 50 mV) to reduce the bipolar current. The third source of leakage is defects in the semiconductor material. The fourth source of leakage is defects at semiconductor/dielectric interfaces. All of these leakage sources can be increased by radiation damage, with the fourth source being especially sensitive to radiation. It is therefore also desirable that a radiation imager comprised of detector diode array have means for reducing the operational degradations caused by the effects of radiation.
The present invention provides, in a first aspect, an imaging apparatus. The imaging apparatus includes a substrate, at least one radiation-sensitive imaging region in the substrate, and a guard region at or immediately adjacent a cut edge of the substrate. The guard region reduces leakage current reaching the radiation-sensitive imaging region from the cut edge when the imaging apparatus is in use. The imaging region(s) and the guard region are electrically reverse biased with respect to the substrate.
The present invention provides, in a second aspect, an imager. The imager comprises a semiconductor substrate, a plurality of light-sensitive imaging regions in the semiconductor substrate, a guard region in the substrate at or immediately adjacent a cut edge of the substrate, and a plurality of electrical contacts for providing a reverse electrical bias to a plurality of light-sensitive imaging devices and a guard device, created, respectively, by the semiconductor substrate together with the plurality of light-sensitive imaging regions and the guard region.
The present invention provides, in third and fourth aspects, methods of fabricating imaging apparatus and imagers of the first and second aspects, respectively.